Endotoxins are lipopolysaccharide fragments from the outer membrane of Gram-negative bacteria. They are released when those bacteria die, which means killing the bacteria does not remove them. They are heat-stable enough to survive autoclaving, they pass through most sterilizing filters, and they are biologically active at picogram quantities.
None of this is detectable by the two techniques that produce the purity number on a certificate.
Why chromatography cannot see them
HPLC and mass spectrometry are configured for peptides. Endotoxins are amphipathic lipopolysaccharides — chemically unrelated to your analyte, poorly behaved on a C18 column, and not producing a recognizable peptide mass. A vial can assay at 99.9% purity and carry an endotoxin load high enough to dominate any immunological readout.
This is the concrete reason a purity figure alone is insufficient. It is not that purity testing is done badly; it is that purity testing was never designed to answer this question.
The LAL assay
The standard method exploits an unusual piece of biology. The blood of the horseshoe crab (Limulus polyphemus) contains amoebocytes that clot on contact with endotoxin. Lysate from those cells — Limulus Amebocyte Lysate, LAL — forms the basis of three assay formats.
Gel-clot is the classic qualitative method: sample and lysate are incubated, and the tube is inverted. A solid clot means endotoxin above the lysate’s sensitivity threshold. Simple, robust, semi-quantitative through serial dilution.
Turbidimetric measures the increase in turbidity as clotting proceeds, with time-to-threshold inversely related to concentration. Quantitative across a broad range.
Chromogenic uses a synthetic substrate cleaved by the activated enzyme cascade to release a coloured product. Absorbance is read at 405 nm. The most precise of the three and the most common on modern certificates.
A recombinant alternative, recombinant Factor C (rFC), replaces crab-derived lysate with an engineered version of the first enzyme in the cascade. It gives comparable sensitivity without depending on a wild animal population, and is increasingly accepted.
Reading the number
Results are reported in endotoxin units per millilitre or per milligram — EU/mL or EU/mg. One EU corresponds to roughly 100 picograms of reference standard endotoxin.
Two things to check on a certificate. First, the units: EU/mL depends on what concentration the sample was tested at, so EU/mg is the more transferable figure for a lyophilized solid. Second, whether a positive product control was run. Peptides can interfere with the LAL cascade in both directions — inhibition produces a falsely low result, enhancement a falsely high one. A spiked recovery control demonstrates the assay actually worked in that matrix.
Endotoxin is ubiquitous on ordinary glassware and in ordinary water. Testing requires depyrogenated labware and endotoxin-free reagents throughout. A low result from a contaminated workflow is meaningless in the same way a high one is.
Where contamination comes from
Endotoxin in a peptide preparation rarely originates in the synthesis, which is a harsh chemical environment. It is introduced later: in the water used for dissolution and buffer exchange, on glassware, during lyophilization, in the filling environment, or from the packaging itself. This is why the test belongs at release, on the final filled vial, rather than on bulk material earlier in the process.
Practical implication
If your work involves cell culture, immune readouts, or anything measuring an inflammatory response, an untested peptide is an uncontrolled variable. Endotoxin activates Toll-like receptor 4 and drives cytokine release on its own. An effect attributed to the peptide may belong to what came in the vial alongside it — and without a number on the certificate, there is no way to rule that out afterwards.